Origin of the Springtime Westerly Bias in Equatorial Atlantic Surface Winds in the Community Atmosphere Model Version 3 (CAM3) Simulation

Abstract

This study makes the case that westerly bias in the surface winds of the National Center for Atmospheric Research (NCAR) Community Atmosphere Model, version 3 (CAM3), over the equatorial Atlantic in boreal spring has its origin in the rainfall (diabatic heating) bias over the tropical South American continent. The case is made by examination of the spatiotemporal evolution of regional precipitation and wind biases and by dynamical diagnoses of the westerly wind bias from experiments with a steady, linearized dynamical core of an atmospheric general circulation model. Diagnostic modeling indicates that underestimating rainfall over the eastern Amazon region can lead to the westerly bias in equatorial Atlantic surface winds. The study suggests that efforts to reduce coupled model biases, especially seasonal ones, must target continental biases, even in the deep tropics where ocean?atmosphere interaction generally rules.